Method and system for optical heterodyne detection of an optical signal

Abstract

An optical heterodyne detection system in accordance with an embodiment of the invention includes two optical receivers for separately measuring the power of an input signal and a local oscillator signal before the signals are combined. The measurements of the input signal and the local oscillator signal are then utilized to enhance the heterodyne signal to noise ratio by removing the intensity noise contributed by the input signal and the local oscillator signal. By measuring portions of the input signal power and the local oscillator signal power and then subtracting out the scaled quantities from the photocurrent measurement during signal processing, the signal to noise of the heterodyne signal is improved beyond that which is accomplished by known balanced receivers.

What is claimed is:  
     
       1. A system for optical heterodyne detection comprising: 
       means for measuring a fraction of a first optical signal before said first optical signal is combined with a second optical signal and for generating a first electrical signal that is representative of said measured fraction of said first optical signal, one of said first and second optical signals being a local oscillator signal and the other signal being an input signal;  
       an optical coupler having a first input and a second input, said first input being optically connected to receive said first optical signal, said second input being optically connected to receive said second optical signal, said optical coupler having an output for outputting a combined optical signal that includes said first optical signal and said second optical signal;  
       a heterodyne receiver having an input for receiving said combined optical signal from said optical coupler and an output for outputting a third electrical signal that is representative of said combined optical signal, said third electrical signal including a heterodyne signal; and  
       a processor for receiving said first electrical signal and said third electrical signal, and for generating an output signal that is indicative of an optical parameter of said input signal in response to said heterodyne signal and said first electrical signal.  
     
     
       2. The system of  claim 1  wherein said processor includes means for utilizing said first electrical signal to reduce signal noise in said third electrical signal that is contributed from said first optical signal. 
     
     
       3. The system of  claim 2  wherein said processor includes means for utilizing said first electrical signal to calculate the signal noise in said third electrical signal that is contributed from said first optical signal. 
     
     
       4. The system of  claim 3  wherein said processor further includes means for subtracting said calculated signal noise related to said first optical signal from said third electrical signal to improve the signal to noise ratio of said heterodyne signal. 
     
     
       5. The system of  claim 1  further including a frequency counter connected to receive a fraction of said local oscillator signal before said local oscillator signal is received by said optical coupler, said frequency counter being connected to transmit a measure of the frequency of said local oscillator signal to said processor. 
     
     
       6. The system of  claim 1  further including means for measuring a fraction of said second optical signal before said second optical signal is combined with said first optical signal and for generating a second electrical signal that is representative of said measured fraction of said second optical signal, said processor further including means for utilizing said second electrical signal to reduce signal noise in said third electrical signal that is contributed from said second optical signal. 
     
     
       7. The system of  claim 1  wherein said optical coupler further includes a second output for outputting a portion of said combined optical signal to said optical receiver, said optical receiver enabling said output signal to be independent of the polarization state of said input signal and balanced with regard to intensity noise of said combined optical signal. 
     
     
       8. The system of  claim 1  further including an optical switch associated with said input signal for blocking said input signal in an optical path that is upstream from said optical coupler. 
     
     
       9. A system for optical heterodyne detection comprising: 
       means for measuring a fraction of an input signal before said input signal is combined with a local oscillator signal and for generating a first electrical signal that is representative of said measured fraction of said input signal;  
       means for measuring a fraction of said local oscillator signal before said local oscillator signal is combined with said input signal and for generating a second electrical signal that is representative of said measured fraction of said local oscillator signal;  
       an optical coupler having a first input and a second input, said first input being optically connected to receive said input signal, said second input being optically connected to receive said local oscillator signal, said optical coupler having an output for outputting a combined optical signal that includes said input signal and said local oscillator signal;  
       a heterodyne receiver having an input for receiving said combined optical signal from said optical coupler and an output for outputting a third electrical signal that is representative of said combined optical signal, said third electrical signal including a heterodyne signal; and  
       a processor for receiving said first electrical signal, said second electrical signal, and said third electrical signal, and for generating an output signal that is indicative of an optical parameter of said input signal in response to said heterodyne signal and said first electrical signal.  
     
     
       10. The system of  claim 9  wherein said processor includes means for utilizing said first electrical signal and said second electrical signal to reduce signal noise in said third electrical signal that is contributed from said input signal and said local oscillator signal, respectively. 
     
     
       11. The system of  claim 9  wherein said processor includes means for utilizing said first electrical signal to calculate the signal noise in said third electrical signal that is contributed from said input signal and for utilizing said second electrical signal to calculate the signal noise in said third electrical signal that is contributed from said local oscillator signal. 
     
     
       12. The system of  claim 11  wherein said processor further includes means for subtracting said calculated signal noise related to said input signal and said calculated signal noise related to said local oscillator signal from said third electrical signal to improve the signal to noise ratio of said heterodyne signal. 
     
     
       13. The system of  claim 12  further including a frequency counter connected to receive a portion of said local oscillator signal before said local oscillator signal is received by said optical coupler, said frequency counter being connected to transmit a measure of the frequency of said local oscillator signal to said processor in order to determine the frequency at which the heterodyne term is detected. 
     
     
       14. A method for monitoring an optical signal utilizing optical heterodyne detection, comprising steps of: 
       providing a first optical signal;  
       providing a second optical signal, one of said first and second optical signals being a local oscillator signal and the other signal being an input signal;  
       measuring a fraction of said first optical signal before said first optical signal is combined with said second optical signal;  
       generating a first electrical signal that is representative of said measured fraction of said first optical signal;  
       combining said first optical signal with said second optical signal to create a combined optical signal;  
       generating a third electrical signal that is representative of said combined optical signal; and  
       generating an output signal that is indicative of an optical parameter of said input signal, said step of generating said output signal including a step of utilizing said first electrical signal to reduce signal noise in said third electrical signal, wherein said signal noise is contributed from said first optical signal.  
     
     
       15. The method of  claim 14  wherein said step of utilizing said first electrical signal to reduce said signal noise includes a step of calculating the noise that is contributed to said third electrical signal from said first optical signal utilizing said first electrical signal and subtracting said calculated noise from said third electrical signal. 
     
     
       16. The method of  claim 14  further including the steps of: 
       measuring a fraction of said second optical signal before said second optical signal is combined with said first optical signal; and  
       generating a second electrical signal that is representative of said measured fraction of said second optical signal.  
     
     
       17. The method of  claim 16  further including steps of: 
       calculating the noise that is contributed to said third electrical signal from said first optical signal utilizing said first electrical signal and subtracting said calculated noise from said third electrical signal; and  
       calculating the noise that is contributed to said third electrical signal from said second optical signal utilizing said second electrical signal and subtracting said calculated noise from said third electrical signal.  
     
     
       18. The method of  claim 14  wherein said step of generating said output signal includes monitoring a heterodyne signal that is a component of said combined optical signal. 
     
     
       19. The method of  claim 14  further including the steps of: 
       blocking transmission of said input signal before said input signal is combined with said local oscillator signal; and  
       generating calibration information that is utilized to generate said output signal.  
     
     
       20. The method of  claim 14  further including a step of sweeping said local oscillator signal across a range of wavelengths in order to monitor said heterodyne signal.